This invention relates to a method of reducing the volatility and decreasing the evaporation loss of water repellent compositions containing a solution of an alkylalkoxysilane and a carrier, for application of the composition to a porous substrate.
Water resistance is an important factor in concrete and masonry construction. This is for the reason that moisture movement in concrete causes or contributes to problems such as expansion, shrinkage, cracking, staining, mildew, lowered resistance to freezing and thawing, chemical attack, corrosion of reinforcing steel, and damage to structures from settling. Because of these problems, various techniques have been used for water repellency including the surface treatment of concrete structures with repellents. Repellents that have been used in the past are oils, waxes, soaps, resins, and silicones, and they have been applied to the masonry surfaces by brush, roller, air spray, or airless spray techniques. One of the most prevalent category of repellent that has been used is silicones, and silicones in organic solvents have been found to be useful for brick, concrete, stucco, or terrazo surfaces. Exemplary of such prior art techniques for rendering concrete water repellent by means of silicones can be found in U.S. Pat. No. 3,772,065, issued Nov. 13, 1973, and U.S. Pat. No. 4,342,796, issued Aug. 3, 1982. A purportedly improved technique is disclosed in U.S. Pat. No. 3,879,206, issued Apr. 22, 1975, where certain additives are included with the silicones. However, in contrast to the latter patent, the technique of the present invention differs in providing for the features of reduced volatility; decreased evaporation loss; the use of a higher molecular weight carrier; the application of a catalyst in addition to a surface depositing agent; an anhydrous solution system; and a step of acidification; all of which are brought out hereinafter in more detail. While the techniques of the prior art can penetrate and thereby render surfaces water resistant, they have not to any substantial degree done anything to block the pores on the surface of the structures since as previously noted, the repellent penetrates into the structure and then cures to a resinous matrix therewithin. The result is that such products do not bead water on the masonry surface in the fashion of the oils, and while the oils do not have the longevity or the penetrating power of silane, for example, the oils are visually striking in their ability to bead and shed surface water. It should also be noted that when alcohol solutions of alkylalkoxysilanes are deposited on a porous surface such as masonry, the solution may penetrate into the porous surface of the masonry or evaporate from that same surface. As this process is occurring during application of the solution to the substrate, the hydrolysis and condensation of the silane is progressing because of the action of water and residual alkalinity in the masonry, as well as because of the presence of catalysts in the solution. However, as the silane is being chemically changed, it penetrates less and evaporates less. It has been found that losses due to evaporation may be as high as seventy to eighty percent of the original weight of silane applied to the substrate. Therefore, and in accordance with the present invention, losses due to evaporation may be reduced by employing alcohol carriers of higher molecular weight. Such alcohols exchange with the methoxy group present in the alkylalkoxysilane to yield a less volatile silane which is capable of penetration into the porous substrate. Further volatility reductions can be achieved by also including in the solution a catalyst to cause the silane to hydrolyze and condense at a more rapid rate, and by adding to the solution a high molecular weight polymer capable of depositing on the surface of the porous substrate. Improved results may be achieved by any of the preceding techniques or combinations thereof depending upon the particular application and conditions. Such disadvantages of the prior art are overcome with the present invention wherein there is provided compositions and methods in which the advantages of both the silane and the oil types of repellents are combined in order to impart to silanes the visual property and affect of water being repelled.